The present invention relates to prosthetic devices and in particular to such devices which deform when in use (eg. when under compressive load). Here particularly the present invention relates to disc nucleus prostheses.
The intervertebral disc comprises three regions, the annulus fibrosus, the nucleus pulposus within the annulus fibrosus, and the cartilaginous end plates. Herniation of the disc may cause the nucleus pulposus to leak through a rupture point in the annulus fibrosus and bear against the ganglia of the spinal nerve, causing severe pain in the back as well as the legs. Surgical procedures to alleviate this condition include excision of the disc, chemo nucleolysis and spinal fusion. Another approach has been to replace the nucleus pulposus with a synthetic material. Thus in U.S. Pat. Nos. 5,047,055 and 5,192,326 it is proposed to replace the nucleus pulposus with a hydrogel in a hydrated form. The axial compression stresses acting upon an intervertebral disc which can vary between 0.15 MPa and 1.5 MPa depending upon the posture and activity involved. With highly hydrated material there is a propensity for the free water with hydrogel to be expelled.
In U.S. Pat. Nos. 4,772,287 and 4,904,206 it is, inter alia, proposed to at least partially replace the annulus pulposus with a prosthesis comprising two capsules arranged side-by-side, each filled with a therapeutic fluid. Similarly in European Patent No.480954 there is disclosed a balloon which is insertable into the cavity formed after removal of nucleus pulposus and which is fillable with fluid thereby to expand within the cavity. Such devices require a valve to prevent excess of the fluid where the disc is under an axial compression load, e.g. by bending.
In U.S. Pat. No. 3,875,595 there is disclosed a collapsible plastic bladder-like prosthesis of generally the same external form as the nucleus pulposus which is provided with a stem such that it can be filled with an unspecified liquid or plastic. The bladder also carries a stud for engagement within a recess formed in the cartilage end-plate or the over/underlying bone of the spinal column to retain the bladder in place. This disadvantage associated with such devices are the surgical procedure needed to form the recess sufficient to receive the stud and the need to prevent excess fluid when the disc is compressed.
The present invention seeks to avoid the disadvantages of the prior art proposals by the provision of a prosthesis which is simpler and cheaper to install and is not subject to mechanical failure.